Hydrocarbons from hydroperoxides

Organic hydroperoxides can be prepared by Hquid-phase oxidation of selected hydrocarbons in relatively high yield. Several cycHc processes for hydrogen peroxide manufacture from hydroperoxides have been patented (84,85), and others (86—88) describe the reaction of tert-huty hydroperoxide with sulfuric acid to obtain hydrogen peroxide and coproduct tert-huty alcohol or tert-huty peroxide. 

The study of the interaction of hydroperoxide with other products of hydrocarbon oxidation showed the intensive initiation by reactions of hydroperoxide with formed alcohols, ketones, and acids [6,134]. Consequently, with the developing of the oxidation process the variety of reactions of initiation increases. In addition to reactions of hydroperoxide with the hydrocarbon and the bimolecular reaction of ROOH, reactions of hydroperoxide with alcohol and ketone formed from hydroperoxide appear. The values of rate constants (in L mol 1 s 1) of these reactions for three oxidized hydrocarbons are given below. 

The reaction of carbon-centered free radicals with O2 is one of the distinguishing features of these intermediates (equations 1, 34, 35). The formation of peroxyl radicals 43 in hydrocarbon oxidation (equation 68) has been of continued interest, " including the important role in biological processes. Peroxyl radicals may also be formed by H atom abstraction from hydroperoxides. 

The selective oxidation of hydrocarbons into hydroperoxides, primary products of oxidation is the most difficult problem because of the high catalytic activity of the majority of applied catalysts in ROOH decomposition. At the same time, the problem of selective oxidation of alkylarens (ethylbenzene and cumene) with molecular 02 in ROOH, is of current importance from the practical point of view in connection with ROOH use in large-tonnage productions such as production of propylene oxide and styrene (a-phenylethylhydroperoxide, PEH), or phenol and acetone (cumyl hydroperoxide) [1],... 

Hydroperoxides decompose relatively slowly at ambient temperatures in the dark, but in light they are readily photolysed to free radicals, (Scheme 1.1, reaction d). Consequently, the rate of photo-oxidation of the hydrocarbon polymers is orders of magnitude higher than thermal oxidation. In addition, small amounts of transition metal compounds, notably iron, cobalt, manganese and copper, have a powerful catalytic effect on radical formation from hydroperoxides [14], leading to rapid molecular weight reduction by breakdown of the intermediate alkoxyl radical and the formation of carboxylic acids and esters as oxidation end products (see Scheme 1.2) [15]. 

In 1956 N.M. Emanuel, Z.K. Maizus, and L.G. Vartanyan, using the kinetic methods of investigation, studied the sequence of transformations of products in oxidized n-decane. The reactions of formation of radicals from hydroperoxides were studied in 1950-60. L. Beitman studied the kinetics of oxidation of several unsaturated compounds and showed in 1953 that ROOH decomposes to radicals by reactions of I and II orders. Several reactions of radical formation were discovered and studied in the sixties the reaction of ROOH with the C— H bond of hydrocarbon (Z.K. Maizus, I.P. Skibida, N.M. Emanuel, 1960), the reaction of ROOH with the carbonyl group of... 

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

Carbonyl compounds can be primary (from radicals or hydroperoxides) or secondary (from alcohols). Thus the picture emerges of hydrocarbon oxidations occurring through compHcated series-sequential pathways as in Figure 1, where clearly other reactions could be going on as well. All possible pathways are pursued to some extent traffic along any pathway is a function of energy requirements and relative concentrations. 

Secondly, the interaction of hindered amines with hydroperoxides was examined. At room temperature, using different monofunctional model hydroperoxides, a direct hydroperoxide decomposition by TMP derivatives was not seen. On the other hand, a marked inhibitory effect of certain hindered amines on the formation of hydroperoxides in the induced photooxidation of hydrocarbons was observed. Additional spectroscopic and analytical evidence is given for complex formation between TMP derivatives and tert.-butyl hydroperoxide. From these results, a possible mechanism for the reaction between hindered amines and the oxidizing species was proposed. 

A potentially powerful probe for sorting out the contribution of hydroperoxide-dependent and mixed-function oxidase-dependent polycyclic hydrocarbon oxidation is stereochemistry. Figure 9 summarizes the stereochemical differences in epoxidation of ( )-BP-7,8-dihydrodiol by hydroperoxide-dependent and mixed-function oxidase-dependent pathways (31,55,56). The (-)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (+)-anti-diol epoxide by both pathways whereas the (+)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (-)-anti-diol epoxide by hydroperoxide-dependent oxidation and to the (+)-syn-diol epoxide by mixed-function oxidases. The stereochemical course of oxidation by cytochrome P-450 isoenzymes was first elucidated for the methycholanthrene-inducible form but we have detected the same stereochemical profile using rat liver microsomes from control, phenobarbital-, or methyl-cholanthrene-induced animals (32). The only difference between the microsomal preparations is the rate of oxidation. 

Oxidation of organic compounds by dioxygen is a phenomenon of exceptional importance in nature, technology, and life. The liquid-phase oxidation of hydrocarbons forms the basis of several efficient technological synthetic processes such as the production of phenol via cumene oxidation, cyclohexanone from cyclohexane, styrene oxide from ethylbenzene, etc. The intensive development of oxidative petrochemical processes was observed in 1950-1970. Free radicals participate in the oxidation of organic compounds. Oxidation occurs very often as a chain reaction. Hydroperoxides are formed as intermediates and accelerate oxidation. The chemistry of the liquid-phase oxidation of organic compounds is closely interwoven with free radical chemistry, chemistry of peroxides, kinetics of chain reactions, and polymer chemistry. 

The enthalpy of the R02 + RH reaction is determined by the strengths of disrupted and newly formed bonds AH= Z>R H—Droo—h- For the values of O—H BDEs in hydroperoxides, see the earlier discussion on page 41. The dissociation energies of the C—H bonds of hydrocarbons depend on their structure and vary in the range 300 - 440 kJ mol-1 (see Chapter 7). The approximate linear dependence (Polany-Semenov relationship) between activation energy E and enthalpy of reaction AH was observed with different E0 values for hydrogen atom abstraction from aliphatic (R1 ), olefinic (R2H), and alkylaromatic (R3H) hydrocarbons [119] ... 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

If radicals are produced in the reactions of unimolecular hydroperoxide decomposition and the reaction of ROOH with hydrocarbon whose concentration at the initial stages of oxidation is virtually constant, the production of radicals from ROOH can be regarded as a pseudo-monomolecular process occurring at the rate V = [ROOH] = + iRH[RH]). The... 

The methods of co-oxidation and oxidation of hydrocarbon (RiH) in the presence of hydroperoxide (ROOH) opened the way to measure the rate constants of the same peroxyl radical with different hydrocarbons. Both the methods give close results [5,9]. The activity of different secondary peroxyl radicals is very close. It is seen from comparison of rate constants of prim-R02 and, v -R02 reactions with cumene at 348 K [9],... 

In the initial period the oxidation of hydrocarbon RH proceeds as a chain reaction with one limiting step of chain propagation, namely reaction R02 + RH. The rate of the reaction is determined only by the activity and the concentration of peroxyl radicals. As soon as the oxidation products (hydroperoxide, alcohol, ketone, etc.) accumulate, the peroxyl radicals react with these products. As a result, the peroxyl radicals formed from RH (R02 ) are replaced by other free radicals. Thus, the oxidation of hydrocarbon in the presence of produced and oxidized intermediates is performed in co-oxidation with complex composition of free radicals propagating the chain [4], A few examples are given below. 

The alkylhydroxyperoxyl and hydroperoxyl radicals formed from alcohol possess a reducing activity and attack hydroperoxide with the formation of the alkoxyl radical. This radical is very active and propagates the chain reacting with hydrocarbon. 

The autoxidation of ethers occurs with self-acceleration as autoxidation of hydrocarbons. The kinetics of such reactions was discussed earlier (see Chapter 2). The autoacceleration of ether oxidation occurs by the initiating activity of the formed hydroperoxide. The rate constants of initiation formed by hydroperoxides were estimated from the parabolic kinetic... 

These data appeared to be very useful for the estimation of the relative O H bond dissociation energies in hydroperoxides formed from peroxyl radicals of oxidized ethers. All reactions of the type R02 + RH (RH is hydrocarbon) are reactions of the same class (see Chapter 6). All these reactions are divided into three groups RO + R (alkane, parameter bre = 13.62 (kJ moC1)172, R02 + R2H (olefin, bre = 15.21 (kJ mob1)1 2, and R02 + R3H (akylaromatic hydrocarbon), hrc 14.32 (kJ mol )12 [71], Only one factor, namely reaction enthalpy, determines the activation energy of the reaction inside one group of reactions. Also,... 

The important role of reaction enthalpy in the free radical abstraction reactions is well known and was discussed in Chapters 6 and 7. The BDE of the O—H bonds of alkyl hydroperoxides depends slightly on the structure of the alkyl radical D0 H = 365.5 kJ mol 1 for all primary and secondary hydroperoxides and P0—h = 358.6 kJ mol 1 for tertiary hydroperoxides (see Chapter 2). Therefore, the enthalpy of the reaction RjOO + RjH depends on the BDE of the attacked C—H bond of the hydrocarbon. But a different situation is encountered during oxidation and co-oxidation of aldehydes. As proved earlier, the BDE of peracids formed from acylperoxyl radicals is much higher than the BDE of the O—H bond of alkyl hydroperoxides and depends on the structure of the acyl substituent. Therefore, the BDEs of both the attacked C—H and O—H of the formed peracid are important factors that influence the chain propagation reaction. This is demonstrated in Table 8.9 where the calculated values of the enthalpy of the reaction RjCV + RjH for different RjHs including aldehydes and different peroxyl radicals are presented. One can see that the value A//( R02 + RH) is much lower in the reactions of the same compound with acylperoxyl radicals. 

Under the conditions where the chain oxidation process occurs, this reaction results in chain termination. In the presence of ROOH with which the ions react to form radicals, this reaction is disguised. However, in the systems where hydroperoxide is absent and the initiating function of the catalyst is not manifested, the latter has a retarding effect on the process. It was often observed that the introduction of cobalt, manganese, or copper salts into the initial hydrocarbon did not accelerate the process but on the contrary, resulted in the induction period and elongated it [4-6]. The induction period is caused by chain termination in the reaction of R02 with Mn"+, and cessation of retardation is due to the formation of ROOH, which interacts with the catalyst and thus transforms it from the inhibitor into the component of the initiating system. 

As alkylaromatic hydrocarbon (toluene, p-xylene, etc.) is oxidized, aldehydes appear radicals and peracids formed from them play an important role. First, aldehydes react rapidly with the Co3+ and Mn3+ ions, which intensifies oxidation. Second, acylperoxyl radicals formed from aldehydes are very reactive and rapidly react with the initial hydrocarbon. Third, aldehydes form an adduct with primary hydroperoxide, which decomposes to form aldehyde and acid. 

The reaction of olefin epoxidation by peracids was discovered by Prilezhaev [235]. The first observation concerning catalytic olefin epoxidation was made in 1950 by Hawkins [236]. He discovered oxide formation from cyclohexene and 1-octane during the decomposition of cumyl hydroperoxide in the medium of these hydrocarbons in the presence of vanadium pentaoxide. From 1963 to 1965, the Halcon Co. developed and patented the process of preparation of propylene oxide and styrene from propylene and ethylbenzene in which the key stage is the catalytic epoxidation of propylene by ethylbenzene hydroperoxide [237,238]. In 1965, Indictor and Brill [239] published studies on the epoxidation of several olefins by 1,1-dimethylethyl hydroperoxide catalyzed by acetylacetonates of several metals. They observed the high yield of oxide (close to 100% with respect to hydroperoxide) for catalysis by molybdenum, vanadium, and chromium acetylacetonates. The low yield of oxide (15-28%) was observed in the case of catalysis by manganese, cobalt, iron, and copper acetylacetonates. The further studies showed that molybdenum, vanadium, and... 

Phenol formed in the system due to acid-catalyzed decomposition of hydroperoxide retards the cumene oxidation. The aqueous phase withdraws phenol from the hydrocarbon phase. This is the reason why the emulsion oxidation of cumene helps to increase the yield of hydroperoxide. The addition of hydrogen peroxide into the system helps to increase the yield of hydroperoxide. 

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